1. Field of the Invention
The present invention relates to bearing systems for supporting shafts in large turbomachinery. More specifically, the present invention relates to the use of a cavitating venturi to control the volume flow rate of lubricant independent of downstream pressure conditions.
2. Description of the Prior Art
Large turbomachinery often includes a large and heavy shaft or rotor supported on a shaft which is mounted in a bearing. In order to commence rotation of a very heavy rotating device, a large torque is required to overcome frictional forces. The shaft in such instances settles on the bearing surfaces and due to its massive weight requires a large startup torque and a large torque to accelerate from low speeds. Additionally, the hydrodynamic lubrication systems designed to provide an oil film about the shaft between the shaft and the bearing during operation at design speeds are not present. The shaft must typically reach a minimum operating speed before lubricant is evenly distributed and an oil film established.
One means of reducing startup torque is to literally jack the shaft from the bearing by pumping oil or other liquid lubricant under pressure through the bearings to create an oil film between the bearing and the shaft. Once the shaft is jacked off the bearing surface, the frictional forces are greatly reduced and the shaft may be rotated with a minimum of effort.
In a typical bearing support system, numerous bearing locations are provided for a large shaft. The lubricant supply system may have a common reservoir and a pump for pressurizing the lubricant to distribute it about the bearing system. Each bearing would be connected to receive liquid lubricant from the pump.
Previous devices have attempted to regulate the volume flow of lubricant to each bearing by using either a capillary tube or a sharp edged orifice to create a flow restriction. One of the problems with either a capillary or a sharp edged orifice is that the volume flow rate therethrough is dependent upon the downstream pressure. Hence, if the particular bearing has a high flow resistance, the feed pressure to the capillary or sharp edged orifice will increase and the volume flow rate will decrease. On the other hand, should there be a reduced flow resistance at the bearing then the volume flow rate through either the capillary or the sharp edged orifice will increase.
Typically, the reservoir of liquid lubricant and the pump size for supplying the liquid lubricant are limited such that the overall quantity of lubricant supplied to the bearing system is limited. If one or more particular bearings is allowed to draw excess lubricant this may either drain the reservoir preventing sufficient lubricant being supplied to other sources or may act to starve one bearing relative to another.
To effectively utilize lubricant for jacking the shaft off the bearing, it is necessary to supply the proper volumetric rate of lubricant flow to each bearing location. A desired volume flow rate may be selected for each location such that lubricant may be supplied at a lower feed pressure and still yield a constant flow response over a wide downstream resistance variation. In a system with each bearing receiving lubricant independently of all others, it is desirable to prevent the flow rate to any bearing from changing even if a lubricant supply to one bearing were to be increased by the downstream resistance dropping to zero.
As disclosed herein, a cavitating venturi is utilized to effect a constant volume flow rate of liquid lubricant through the venturi relatively independent of the system resistance. The cavitating venturi acts to allow a predetermined volume flow rate to pass therethrough relatively independent of the downstream resistance.
The present invention is described relative to the use of a cavitating venturi to regulate and balance the volume flow rates to numerous bearings within a bearing system for effecting essentially hydrostatic operation. This valve could also be used in a hydrodynamic mode during which the shaft is operating at design speeds to regulate the volume flow rate of lubricant being supplied at any particular location.